1. Field of the Invention
This invention relates to signal probe apparatus for use with a signal measuring device, such as an oscilloscope or the like.
2. Description of the Prior Art
In the prior art, signal probes are normally used with a signal measuring device, such as an oscilloscope, for measuring signals at desired locations in a circuit. Through the use of appropriate probe tips, these probes may be easily secured to the desired locations. Such probes may be intended for use with one or more particular oscilloscopes wherein certain considerations, such as those relating to bandwidth and to load termination between the probe and the oscilloscope, have already been taken into account. Further, resistive cabling and/or proper shielding may be designed into the probes so as to minimize the amount of stray signals which may otherwise be mixed with the desired signals to be measured and thus detected. Nevertheless, probes may insert a load into the circuit which may affect the signal to be measured. For example, when measuring relatively high frequency signals, circuit loading in the form of additional capacitance may be inserted by the probe. To minimize this additional capacitance, attenuation probes, such as a voltage sensing passive type, may be used. These probes attenuate signals by a predetermined attenuation amount, for example, 10X or 100X, and, as a result, reduce or minimize the induced additional capacitance. However, in so doing, the signal being measured is also attenuated by the same amount.
An example of an attenuation probe 10 is illustrated in FIG. 1A. As shown therein, the attenuation probe 10 generally comprises a main body 12, a cable 18, a compensation portion 20 and a connector 22. The main body 12 includes a probe tip 14, an attenuation portion 52 (FIG. 1B) and a switch 16. Further, a ground wire 11 having a ground attachment 13, for example, an alligator-type clip, is coupled to the main body 12. As is to be appreciated, in measuring high frequency signals, it may be desirable to minimize the length of the ground wire 11.
The compensation portion 20, which contains an adjustable portion 54 having an adjustable capacitor 50 (FIG. 1B) or similar device, is adapted to compensate for variations in the oscilloscope input capacitance. The probe 10, and particularly the compensation portion 20, further includes an element or elements, such as a sense resistor 24 (FIG. 1B), which is utilized to provide an indication to the oscilloscope of the amount of attenuation (i.e. the attenuation factor) associated with the probe.
To measure a signal at a desired test point, the probe tip 14 is placed on such test point and the ground clip 13 is securely attached to an acceptable ground connection. The signal from the test point is supplied from the probe tip 14 by way of a first wire or signal transmission line contained within the probe 10 through the attenuation portion 52 and the cable 18 to the adjustment portion 54 of the compensation portion 20. Similarly, ground is coupled from the ground attachment 13 through the ground wire 11 to the main body 12 and from there by way of a second (ground) line contained within the probe 10 through the cable 18 to the adjustment portion 54 of the compensation portion 20. As shown in FIG. 1B, a third signal line (commonly referred to as a sense line) is coupled to the ground line in the main body 12 through the sense resistor 24 included in the compensation portion 20 and is adapted to provide an indication or sense signal therefrom. The measured signal, ground signal and sense signal are thereafter supplied from the compensation portion 20 to the connector 22 which is adapted to mate with a corresponding connector on the oscilloscope. As a result, when the connector 22 is connected to the corresponding mating connector of the oscilloscope, the signals from the attenuation probe 10 are supplied to the oscilloscope.
In certain situations, for example, when using multiple probes which provide multiple signal displays, it may be desirable for the operator to confirm the origin of a signal currently being displayed by the oscilloscope. In these situations, the operator may press (close) the switch 16 of a probe, which is preferably a momentary-type switch, to short circuit, or by-pass, resistor 24, thereby changing the resistance between the ground line and the sense line, as hereinafter more fully described. Upon sensing such change in resistance, the oscilloscope typically displays an identification mark on its display screen, thereby providing an indication to the operator that the signal being displayed is obtained from the respective probe in which the switch 16 was pressed.
FIG. 1B illustrates a schematic diagram of the probe 10. In FIG. 1B, the probe tip 14 and the ground clip 13 correspond to terminals 26 and 28, respectively. The terminal 26 is coupled by way of the signal transmission line through the attenuation portion 52 and the adjustment portion 54 to an output terminal 30 contained within the connector 22. Similarly, the terminal 28 is coupled by way of the ground line through the adjustment portion 54 to a terminal 32 contained within the connector 22. A terminal 34, also contained within the connector 32, is coupled to the sense line which is connected to the ground line by way of the sense resistor 24. Further, the switch 16 is connected across the resistor 24 as shown in FIG. 1B, to selectively by-pass the resistor.
The attenuation portion 52 may include resistors 40 and 42 and a capacitor 44 connected as shown in FIG. 1B. The values of these components are dependent upon the amount of desired attenuation of the signal to be measured, that is, the signal applied to the probe tip (or to terminal 26). As an example, for a probe having an attenuation factor of X10 (i.e. V.sub.out =V.sub.in /10) for use with an oscilloscope having a 1 Mohm input resistance, these components may have the following values:
resistor 40:47 ohms PA1 resistor 42: 9 Mohms PA1 capacitor 44:12 pf
The adjustment portion 54 may include the adjustable capacitor 50 and resistors 46 and 48 connected as shown. The adjustable capacitor 50 may be adjusted by the operator so as to compensate for variations in the input capacitance of the oscilloscope, as previously discussed. As is to be appreciated, portions 52 and 54 may be configured differently from that shown in FIG. 1B and may be located in different locations of the probe 10. For example, the attenuation portion 52 may be located in the compensation portion 20.
The value of the resistor 24 identifies the attenuation factor associated with the probe 10. In other words, probes performing different levels of signal attenuation will have different respective resistance values for the resistor 24 so as to produce different respective values or ranges of values of a sense signal across the terminals 32 and 34 as, for example, those shown in Table 1.
TABLE 1 ______________________________________ Probe Attenuation Factor R.sub.32-34 (ohms) ______________________________________ 1 X1 &gt;15K 2 X10 11K +/- 10% 3 X100 5.6K-6.2K +/- 10% ______________________________________
As shown therein, each of three different attenuation probes utilizes a value or range of values for the resistor 24 which is unique from the others. Further, such resistance values are sufficiently separated from one another so as to allow for component and measuring tolerances. As a result, by measuring the resistance across terminals 32 and 34 when the switch 16 is opened, the oscilloscope is provided with an indication of the amount of signal attenuation performed by the respective probe. For example, if a conventional sense, or measuring circuit in the oscilloscope detects a resistance R.sub.24 across the terminals 32 and 34 of approximately 11K ohms, this indicates that the probe being utilized has an attenuation factor of 1OX and a predetermined amount of compensation is provided for such attenuation. For example, horizontal and/or vertical sensitivity may be adjusted accordingly. On the other hand, when the switch 16 is closed (activated), the resistance across the terminals 32 and 34 is effectively a short circuit. In this situation, the sense or measuring circuit interprets such low resistance as a command to display an identification mark on the display screen, as hereinbefore discussed.
Typically, the resistance R.sub.24 across the terminals 32 and 34 is sampled by the sense or measuring circuit at a predetermined sampling rate. The sampled resistance information, in analog form, is supplied to an analog-to-digital converter contained within the oscilloscope (not shown) so as to be converted into digital form. Thereafter, the digital information signal is supplied to a processor (not shown) which processes the information in accordance with a previously stored algorithm so as to compensate for the attenuation factor.
Although the above-described probe 10 reduces capacitive loading and provides an identification signal, such probe may be cumbersome to use in taking signal measurements and particularly when different functions performable by a multi-function oscilloscope are desired. For example, consider the situation in which an operator, who is working alone, is holding a probe in each hand on two respective test points and wishes to change or activate one or more functions of the oscilloscope. In such a situation, the operator typically frees one of his or her hands, thereby interrupting the measuring of the signals, so as to adjust the appropriate knob or activate a particular switch on the oscilloscope. The prior art has failed to provide a means for remotely controlling one or more functions of an oscilloscope which is convenient for an operator to use while actively measuring signals.